Selectively movable shift forks are employed in vehicular transmissions to translate the means employed therein to select the desired gear ratio between the engine and the drive wheels of the vehicle. The present invention is directed to an improved configuration for shift forks, and particularly the contact pads or shoes presented from the shift fork, to engage whatever members are utilized within the transmission to effect selection of the desired gear ratio. However, before proceeding to describe the invention itself, it may be helpful to provide a brief overview of the environment within which the invention is to be employed. This can begin with a brief description of a representative vehicular transmission.
A transmission is incorporated in a vehicular drive train between the engine and the drive wheels to provide a plurality of gear ratios for moving forwardly, and generally one gear ratio for moving rearwardly. The transmission is a necessary part of a vehicular drive train, because internal combustion engines can deliver only limited torque at low revolutions per minute (RPM). The transmission allows the speed of the engine to be maintained within its optimum operating range for the delivery of maximum torque or power as the vehicle accelerates from a stationary or "stopped" position to the desired speed. The speed reduction between the RPM of the engine and the resulting rotation of the drive wheels provided by a transmission effects a controlled application of the torque by which the drive wheels are rotated. Accordingly, when that gear ratio, commonly designated as "low" or "first" gear, is selected, the transmission imparts less speed to the drive wheels, but imparts more torque from the engine to rotate the drive wheels in order to overcome the static inertia of the vehicle and thereby initiate forward movement. As the speed of the vehicle increases, the transmission may be selectively shifted through the plurality of gear ratios provided by the transmission in order to impart progressively greater rotational speed to the drive wheels with concomitantly lesser torque.
Many manual transmissions provide three forward gear ratios. These transmissions are generally identified as "three-speed" transmissions, but "four-speed" and "five-speed" transmissions are also quite common, because they permit an engine to operate within a smaller, optimal speed range, while effecting progressively increasing forward speed to the vehicle. Irrespective of the exact number of gear ratios provided, manual transmissions offer a plurality of forward speed gear ratios from which the driver may make the desired selection--though normally the selection is sequential--to transmit torque from the engine to the drive wheels. Generally, only a single reverse gear ratio is provided.
In a typical three-speed transmission there is generally an input shaft which is operatively rotated by the engine of the vehicle. The input shaft generally has a clutch gear or synchronizer assembly connected thereto so that rotation of the input shaft always rotates a portion of the clutch gear.
The transmission also has an output or transmission shaft which is operatively connected to the drive wheels of the vehicle, as through a differential. In a representative three-speed manual transmission there are generally two gears which are operatively carried by the output shaft to provide a means by which to rotate the output shaft. The driving connection between the two gears and the output shaft is quite often achieved by a spline connection, so that the gears may be selectively translated axially along the output shaft while maintaining a constant rotational driving connection therewith. The two gears so supported on the output shaft are also of preferably different diameters.
The representative transmission also has a countershaft upon which a number of gears are operatively supported. One of those gears is normally affixed to the countershaft and constantly meshed with the clutch gear in order to effect rotation of the countershaft in response to rotation of the input shaft.
In the representative three-speed transmission being discussed, each of the two gears on the output shaft interact with a shift fork--a bracket that is movable to effect selective engagement or disengagement between those gears, and one or the other of the gears on the countershaft.
In low gear, the larger of the two gears on the output shaft is operatively engaged with the smaller gear on the countershaft. When the engine approaches the upper limit of its operating range, the driver can shift into the second gear ratio by manipulating a shift fork to engage the smaller diameter gear on the output shaft with the larger gear on the countershaft. For cruising speed, the driver shifts into the third gear ratio. This may be accomplished by using a shift fork to force the smaller of the gears on the output shaft axially into engagement with the clutch gear. To select reverse gear, a small gear on an idler shaft is interposed between a gear on the countershaft with a gear on the output shaft, again by movement of an appropriate shift fork.
In some transmissions the gears are not fixed to the shaft on which they are carried. In those installations, the shift fork translates a member--such as the coupling collar typically employed in a shift synchronizer--selectively into and out of engagement with a gear that otherwise rotates freely on its supporting shaft. Engagement of the coupling sleeve with the gear secures the gear to the shaft on which the gear is rotatably supported so that they rotate in unison. Such a transmission generally employs multiple shift rails mounted within the transmission casing for axial translation. The shift rails support the necessary shift forks, which selectively translate the coupling collar in the appropriate shift synchronizer to effect a driving connection between the gear and the shaft on which it is carried--usually either the countershaft or the output shaft.
Irrespective of how the gears are connected to their supporting shafts, contact surfaces on the shift fork are slidably received within an annular groove or race provided in the member to be translated, be it the gear or the coupling collar. The shift fork is, itself, supported by a hub portion. The hub portion may be slidably received on a mounting shaft; it may be secured to an axially translatable shift rail; or, it may be pivotally mounted on a support shaft. Irrespective of how the hub portion is supported, an offset arm extends radially outwardly from the hub portion, and the outer end portion of the offset arm may be bifurcated into two, divergent arms or tines which embrace the race in the gear or coupling collar to be translated by that shift fork.
In the vast majority of manual transmissions, when the driver operates a shift lever to effect a change in the gear ratio, the movement of the shift lever is communicated to a shift fork that is utilized to engage or disengage the necessary gears, either with each other or with the shafts on which they are carried. As such, it is commonly thought that the shift fork must be sufficiently rigid to overcome the resistance offered by the interaction of those components within the transmission which effect selection of the gear ratio. This conclusion has been empirically determined on the basis that deflection of the prior art shift forks has produced deleterious results. Specifically, deflection of a prior art shift fork can: (1) cause the coupling collar to "cock" on its shaft; (2) cause the independently mounted contact pads interposed between the shift fork and the element engaged thereby to "cock"; and/or (3) cause the contact pads of prior known configuration to wipe away the protective lubricant interposed between each contact pad and the transmission which it engages.
Shift forks are made from a wide variety of materials to accomplish the desired design parameters. As should now be apparent, the principal design parameters have heretofore been that the hub and offset arm portions of the shift fork should resist the bending stresses to which they are typically exposed. At the same time, the ends of the bifurcated tines which come into contact with the race, in either the gears themselves or coupling collars, must have a low coefficient of friction and yet good resistance to wear.
Shift forks have been made by casting a copper-aluminum alloy which makes it possible not only to impart the desired frictional characteristics to the ends of the shift fork, but also provide the necessary structural resistance heretofore deemed to be required. The use of brass offers the same advantages. However, these materials are quite heavy, and they are relatively expensive. Moreover, the mass of material required to impart the desired strength not only increases the weight of the shift fork, but also increases the size thereof, so that they become a significant factor in designing the transmission housing. Shift forks made from cast iron or steel can certainly provide the desired rigidity, but prior known designs for the shift forks have led to the conclusion that such materials cause degradation of the shift forks themselves or of the race with which the shift fork interacts. It has, therefore, been necessary to provide some metallurgical treatment to the shift forks in order to obviate frictional wear and degradation. Alternatively, it has heretofore been suggested that the use of separate wear inserts could accomplish the desired wear resistance. The use of separate contact pads does make it possible to fabricate the shift fork itself out of a stress resistant material and yet make the contact pad out of a wholly distinct material which has the requisite wear and friction characteristics. However, even though it has been recognized that the use of separate contact pads can be beneficial, it has not been appreciated that the planar or arcuate configurations heretofore employed tend to wipe away the lubricant if the shift fork is not sufficiently rigid to preclude deflection under the anticipated loading.
As such, it has not previously been thought possible to lighten the shift fork and allow it to deflect under the forces to which it is commonly subjected in a transmission, and at the same time rely upon the configuration of the contact pad to accommodate the deflection and thereby avoid the operational degradation heretofore experienced when the shift fork has been inadvertently permitted to deflect.